Stop Order is an order to buy securities at a price above or sell at a price below the current market. Stop buy orders are generally used to limit loss or protect unrealized profits on a short sale. Stop sell orders are generally used to protect unrealized profits or limit loss on a holding. A stop order becomes a market order when the stock sells at or beyond the specified price and, thus, may not necessarily be executed at that price.
Investors commonly use a stop order before leaving for holidays or entering a situation where they are unable to monitor their portfolio for an extended period. Stops are not a 100% guarantee of getting the desired entry/exit points. For instance, if a stock gaps down, the trader's stop order will be triggered (or filled) at a price significantly lower than expected.